Polysilicon semiconductor thin film substrate, method for producing the same, semiconductor device, and electronic device

ABSTRACT

A semiconductor device and a method of manufacture thereof by forming an amorphous semiconductor film on the surface of an insulative substrate, and irradiating the amorphous semiconductor film with a laser beam to crystallize it to form a polycrystalline semiconductor thin film. A transistor is then formed in the polycrystalline semiconductor thin film. More specifically, a UV-ray is irradiated to the rear face of the insulative substrate or the amorphous semiconductor film to heat the amorphous semiconductor film to a melting temperature or lower. Then a laser beam at a suitable shape selection laser energy density Ec forms the crystal grains with the number of closest crystal grains of 6 most predominantly being irradiated to convert the amorphous semiconductor film into a polycrystalline semiconductor thin film. The thin film transistor formed in this structure has a high yield and is capable of high-speed operation.

TECHNICAL FIELD

The present invention relates to a polycrystalline semiconductor thinfilm substrate, a manufacturing method thereof, a semiconductor device,a method of manufacturing a semiconductor device and an electronicapparatus and, more in particular, it relates to a technique which iseffective to application use for manufacturing a transistor (thin filmtransistor: TFT) in a surface layer portion of a polycrystalline film(polycrystalline semiconductor thin film) and a manufacturing techniqueof a polycrystalline semiconductor thin film substrate for manufacturingthe thin film transistor, as well as an electronic apparatusincorporated with the thin film transistor such as a liquid crystaldisplay or data processor.

BACKGROUND ART

Thin film transistors (TFT) used for image displays in the prior arthave been formed using, as a device material, polycrystalline siliconformed by a melting recrystallization method such as excimer laserannealing from amorphous silicon or microcrystalline silicon formed by aplasma CVD process on an insulative substrate such as glass or quartz asa base material.

The semiconductor device (TFT) and the manufacturing method thereof inthe prior art are to be described with reference to FIG. 17( a) to (d).As shown in FIG. 17( a), an amorphous silicon thin film 202 is depositedon one surface of a glass substrate 201.

Then, as shown in FIG. 17( b), when the surface of the amorphous siliconthin film 202 is scanned by a linear excimer laser beam 204 in thedirection of an arrow 203, the amorphous silicon thin film 202 is heatedby the excimer laser beam 204 and changed from an amorphous structureinto a polycrystalline structure. When the entire surface of theamorphous silicon film 202 is heated by the excimer laser beam 204 underscanning, a polycrystalline silicon thin film 205 is formed as shown inFIG. 17( c). In FIG. 17( c), the polycrystalline silicon thin film 205is made of silicon crystal grains and a grain boundary 206 is formedbetween the crystal grains.

The process described above is referred to as a laser heating process.This is adopted when a polycrystalline silicon thin film at high qualityis prepared on a substrate comprising a low melting material such asglass. They are described in details, for example, in “1996 Society forInformation Display International Symposium Digest of Technical Papers,pp 17-29” and “IEEE Transactions on Electron Devices, vol. 43, No. 9,1996, pp 1454 to 1458” and the like.

FIG. 17( d) shows a transistor (TFT) formed by using the polycrystallinesilicon thin film in FIG. 17( c).

A gate insulation film 208 such as a silicon oxide film is disposed onthe polycrystalline silicon thin film 205. Further, a source impurityimplantation region 207 and a drain impurity implantation region 209 areformed on the polycrystalline silicon thin film 205. A thin filmtransistor is formed by disposing a gate electrode on thesource-impurity implantation regions 207 and 209 and the gate insulationfilm 208.

FIG. 18 shows the dependence of the size of the silicon crystal grainand the roughness of the polycrystalline silicon thin film on theirradiation laser energy in the prior art (dependence 301 of the size ofthe crystal grain on the laser energy density). Silicon is notcrystallized at the energy with a laser energy density of 200 mJ/cm² orless but crystallization initiates when it exceeds 200 mJ/cm² and thesize of the crystal grain increases along with an increase in the laserenergy density.

However, when the laser energy density exceeds 250 mJ/cm², the siliconcrystal grain becomes smaller. Since a polycrystalline silicon thin filmtransistor having favorable characteristics may be manufactured byincreasing the size of the silicon crystal grains, the energy density ofthe laser is set to 250 mJ/cm².

The value of the laser energy density in the prior art may sometimesvary since it depends on the nature of the amorphous silicon film (forexample, growing method, film thickness). They are described in details,for example, in “Applied Physics Letters, vol. 63, No. 14, 1993, pp1969-1971”.

Further, for increasing the grain diameter of the crystal grains, laserirradiation may preferably be conducted by heating the substrate at 400°C. This is because the solidification velocity is lowered by heating thesubstrate and the grain diameter increases up to about 500 nm. Further,since a temperature gradient is caused at the end of the laser beam, thesize of the crystal grains varies remarkably. In order to prevent this,laser may be preferably irradiated under overlapping. They are reportedin “Proceedings of The Institute of Electronics, Information andCommunication Engineers C-II Vol. J76-C-II, 1993, pp 241-248”.

Further, for making the size of the crystal grains uniform, first laserirradiation is applied at first at a low energy density and,subsequently, a second laser irradiation is applied at a high energydensity required for crystallization. Such a two-step laser irradiationis applied for forming crystal seeds by the first laser irradiation andcrystallization of them by the second laser irradiation. In this case,while the uniformess is improved, the crystal grain diameter is reduced.This is reported in “Proceedings of 42th Laser Materials ProcessingConference, 1997, pp. 121-130”.

DISCLOSURE OF THE INVENTION

It has been found that the prior art described above involves thefollowing problems.

When many grain boundaries are present in a silicon channel region undera gate electrode, non-uniformess thereof may sometimes lower the carriermobility μ to several cm²/V·s, for example, due to the variation ofconduction carriers.

Further, when the grain boundaries present in the silicon channel regionunder the gate electrode varies in density, a threshold voltage V_(th)varies up to several V in individual transistors.

In addition, when the crystal grain in the silicon channel region underthe gate electrode varies in size, individual transistors vary incarrier mobility μ.

Further, when roughness of the grain boundaries is present in thesilicon channel region under the gate electrode, individual transistorsvary and degrade in performance.

Further, when impurities are implanted in the polycrystal region, sinceimpurities segregate in the grain boundaries, it is difficult to controlthe carrier concentration.

The present inventors have made observation and studies on thedistribution of crystal grains of polycrystal semiconductor thin filmsmanufactured in the prior art. FIG. 19 shows the arrangement of crystalgrains of the polycrystalline semiconductor thin film in the existentpolycrystalline semiconductor thin film substrate used for themanufacture of thin film transistors.

This figure is based on microscopic photography and crystal grains 250are in various shapes such as trigonal, tetragonal, pentagonal,hexagonal, heptagonal and octagonal shapes, in which hexagonal crystalgrains 251 are most predominant. It has been found that the number ofthe hexagonal grains 251 is about 30 to 40%. A square region of 10 μmside is taken as an evaluation region and observation was made at anoptional place.

The present inventors have considered that the crystal grains can beunified in the polycrystal semiconductor thin film, that is, thecharacteristics of the thin film transistor can be improved and unifiedby decreasing the number of trigonal, square and pentagonal shapes andincreasing the number of hexagonal crystal grains by so much.

Then, when a relation between the energy density of the laser irradiatedto the amorphous silicon film and the shape of the crystal grains formedwas examined, it has been found that there exists a laser energy densityon every shape that maximizes the formation of each shape (suitableshape selection laser energy density Ec). That is, it has been foundthat there exist laser energy density that maximizes formation of thetetragonal shape, the laser energy density that maximizes formation ofthe pentagonal shape and the laser energy density that maximizesformation of the hexagonal shape.

This invention is an invention adopting a crystallization method basedon the suitable shape selection laser energy density Ec by the findingdescribed above, which defines the crystal grains in the polycrystallinesemiconductor thin film as the hexagonal shape and defines the rate ofthe hexagonal shape to be 50-100%.

An object of this invention is to provide a polycrystallinesemiconductor thin film in which the size of the crystal grains and thecarrier concentration are uniform and which has a planar surface.

Another object of this invention is to provide a semiconductor devicehaving a thin film transistor with favorable characteristics and withless variation in characteristics.

A further object of this invention is to provide an electronic apparatuswith favorable characteristics incorporated with a semiconductor devicehaving a thin film transistor.

The foregoing and other objects, as well as novel features of thisinvention will become apparent with reference to the descriptions of thespecification and the appended drawings.

Outline of typical inventions among those disclosed in the presentapplication will be described simply as below.

The means (1) described above provides a polycrystal semiconductor thinfilm substrate comprising an insulative substrate and a polycrystallinesemiconductor thin film disposed on one surface of the insulativesubstrate, in which 50-100% of the crystal grains forming thepolycrystalline semiconductor thin film are of a hexagonal shape.Electron orbits of the crystal grains at the surface and inside thepolycrystalline semiconductor thin film are bonded to each other. Theroughness of the grain boundaries on the surface of the polycrystallinesemiconductor thin film is 5 nm or less. The insulative substrate is aglass substrate and the polycrystalline semiconductor thin film is apolycrystalline silicon film.

Such a polycrystalline semiconductor thin film substrate is manufacturedby the following method. In a method of manufacturing a polycrystallinesemiconductor thin film substrate by forming an amorphous semiconductorfilm on the surface of an insulative substrate, then irradiating theamorphous semiconductor film with a laser beam to crystallize theamorphous semiconductor film to form a polycrystalline semiconductorthin film, a UV-ray is irradiated to the rear face of the insulativesubstrate or to the amorphous semiconductor film thereby to heat theamorphous semiconductor film to a melting temperature or lower, and alaser beam at a suitable shape selection laser energy density Ec whichmaximizes formation of the crystal grains hexagonal shape is irradiatedrepeatedly to the surface of the amorphous semiconductor film, therebyconverting it into the polycrystalline semiconductor thin film.

After repeating a first laser irradiation for multiple cycles at thesuitable shape selection laser energy density Ec to the surface of theamorphous semiconductor film, a second laser irradiation is repeated formultiple cycles at an energy density lower than that of the laser energydensity Ec. The first laser irradiation and the second laser irradiationare conducted while scanning the laser beam along the surface of theamorphous semiconductor film. The irradiation is conducted whilesynchronizing the period of the laser beam irradiation and the period ofthe UV-ray heating. The laser beam irradiation is conducted by anexcimer laser and the laser beam emitted from the excimer laser isdivided by an optical component into two optical channels, such that oneof them reaches a laser beam irradiation position with a delay and thelaser beam passing through a shorter optical channel length isattenuated by being passed through an optical attenuator and caused toreach the laser beam irradiation position, thereby forming thepolycrystalline semiconductor thin film.

(2) A semiconductor device in which plural transistors are formed to apolycrystalline semiconductor thin film wherein the transistor (thinfilm transistor) is formed to a polycrystal semiconductor thin film ofthe constitution (1) described above.

The semiconductor device as described above is manufactured by thefollowing method. It is manufactured by a method of manufacturing asemiconductor device by forming plural transistors in a polycrystallinesemiconductor thin film wherein the polycrystalline semiconductor thinfilm has the constitution (1) described above.

(3) An electronic apparatus incorporating a semiconductor device inwhich plural transistor are formed in a polycrystal semiconductor thinfilm, wherein the semiconductor device is made up with the semiconductordevice of the constitution (2) described above. For example, theelectronic apparatus is a liquid crystal display, and the semiconductordevice has transistors for operating each of pixels of a liquid crystaldisplay panel and transistors constituting the peripheral drivercircuitries and is attached being overlapped to a liquid crystal displaypanel of the liquid crystal display.

(4) An electronic apparatus incorporating a semiconductor device inwhich plural transistors are formed in a polycrystalline semiconductorthin film, wherein the electronic apparatus is, for example, a dataprocessor in which a central processing unit, a cache circuitry, amemory circuitry, a peripheral circuitry, an input/output circuitry anda bus circuitry are formed with each of the transistors of thesemiconductor device.

According to the constitution (1) described above, (a) since 50 to 100%of the crystal grains in the polycrystalline semiconductor thin filmcomprise hexagonal crystal grains and the grain diameter is uniform asfrom 0.2 to 0.3 μm, a substrate with improved carrier mobility μ andwith less variation in the carrier mobility μ in each of the regions canbe provided.

(b) Since the electron orbits of the grain boundaries at the surface andinside the polycrystalline semiconductor thin film are bonded to eachother, it is possible to provide an effect of making the carriermobility constant and improving the reliability of individualtransistors. That is, longer life can be attained for individualtransistors.

(c) Since the laser beam is irradiated repeatedly upon formation of thepolycrystalline semiconductor thin film, roughness on the surface of thepolycrystalline semiconductor thin film can be reduced to provide aplanar polycrystalline semiconductor thin film substrate.

(d) Since laser beam irradiation is conducted repeatedly by multiplecycles at the suitable shape selection laser energy density Ec mostpreferred for forming the hexagonal shape, hexagonal shape crystals areformed successively in the amorphous semiconductor film, adjacenthexagonal crystal grains move to each other and are gradually in closecontact with adjacent hexagonal crystal grains. Subsequently, sincelaser beam irradiation is conducted repeatedly for multiple cycles at alaser energy density lower than the suitable shape selection energydensity Ec, less impurities less-segregate to the grain boundaries tomake the carrier concentration of the crystal grains constant.

According to the constitution (2) described above, (a) since 50 to 100%of the crystal grains comprise hexagonal crystal grains and the grainsize thereof is uniform as 0.2 to 0.5 μm in the polycrystallinesemiconductor thin film, when a transistor (TFT) is formed, the grainboundaries are reduced in the silicon channel region under the gateelectrode, the carrier mobility μ is improved and variation in thecarrier mobilities is decreased in each of the transistors.

(b) In each of the transistors, the density of the grain boundariespresent in the silicon channel region below the gate electrode variesless and the threshold voltage V_(th) is made uniform in each of thetransistors.

(c) Since the laser beam is irradiated repeatedly upon forming thepolycrystalline semiconductor thin film, roughness on the surface of thepolycrystalline semiconductor thin film is reduced and variation in theperformance of individual transistors is reduced and less degradationoccurs to attain the longer life of the transistors.

(d) In the formation of the polycrystalline semiconductor thin film,since laser beam irradiation is conducted repeatedly for multiple cyclesat the suitable shape selection laser energy density Ec most preferredfor forming the hexagonal shape, hexagonal seed crystals are formedsuccessively in the amorphous semiconductor film, adjacent hexagonalcrystal grains move to each other and are gradually in close contactwith adjacent hexagonal crystal grains. Subsequently, since laser beamirradiation is conducted repeatedly for multiple cycles at a laserenergy density lower than the suitable shape selection energy densityEc, less impurities segregate to the grain boundaries to make thecarrier concentration of the crystal grains constant. As a resultcharacteristics of the transistors become stable.

According to the constitutions (3) described above, since each of pluraltransistors for operating each of pixels on the liquid crystal displaypanel is made uniform in the characteristics, image at high quality canbe obtained.

According to the constitution (4) described above, since the centralprocessing unit, the cache circuitry, the memory circuitry, theperipheral circuitry, the input/output circuitry and the bus circuitryare formed with the thin film transistors formed on the glass substratesurface, it is possible to provide a data processor of reduced thicknessand higher performance.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross sectional view illustrating a state ofmanufacturing a polycrystalline semiconductor thin film according to anembodiment (Embodiment 1) of the present invention.

FIG. 2 is a schematic cross sectional view illustrating a method ofmanufacturing a polycrystalline semiconductor thin film of Embodiment 1.

FIG. 3 is a perspective view of a polycrystalline semiconductor thinfilm substrate of Embodiment 1.

FIG. 4 is a partial cross sectional view of a polycrystallinesemiconductor thin film substrate of Embodiment 1.

FIG. 5 is a schematic plan view illustrating the constitution of crystalgrains in a polycrystalline semiconductor thin film of a polycrystallinesemiconductor thin film substrate of Embodiment 1.

FIG. 6 illustrates a group of graphs showing the result of analysis suchas properties of crystal grains and difference in the manufacturingconditions in the manufacture of a polycrystalline semiconductor thinfilm.

FIG. 7 is a graph showing a correlation between hexagonal crystal grainsand the number of cycles of laser beam irradiation in the manufacture ofa polycrystalline semiconductor thin film and a graph illustrating arelation between the number of cycles of laser beam irradiation and theroughness on the surface of a polycrystalline semiconductor thin film.

FIG. 8 is a schematic view illustrating the difference of crystal grainsdepending on the difference in a laser energy density.

FIG. 9 is a schematic view illustrating the growing process of crystalgrains by laser beam irradiation applied repeatedly.

FIG. 10 is a schematic cross sectional view illustrating a transistor(thin film transistor) manufactured according to Embodiment 1.

FIG. 11 is a schematic cross sectional view illustrating a method ofmanufacturing a thin film transistor according to Embodiment 1.

FIG. 12 is a schematic cross sectional view illustrating the state ofmanufacturing a polycrystalline semiconductor thin film according toanother embodiment (Embodiment 2) of the present invention.

FIG. 13 is a schematic cross sectional view illustrating the state ofmanufacturing a polycrystalline semiconductor thin film according toanother embodiment (Embodiment 3) of the present invention.

FIG. 14 is a schematic perspective view illustrating a portion of aliquid crystal display according to another embodiment (Embodiment 4) ofthe present invention.

FIG. 15 is a schematic perspective view illustrating a portion of a dataprocessor according to another embodiment (Embodiment 5) of the presentinvention.

FIG. 16 is an enlarged schematic view of a portion surrounded with adotted chain in FIG. 15.

FIG. 17 is a schematic cross sectional view illustrating an existentmethod of manufacturing a thin film transistor.

FIG. 18 is a diagram illustrating a correlation between the laser energydensity and the crystal grain diameter in the prior art.

FIG. 19 is a schematic view illustrating a constitution of crystalgrains of a polycrystalline semiconductor thin film in an existentpolycrystalline semiconductor thin film substrate.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, preferred embodiments of this invention will be describedwith reference to the drawings, in which like or corresponding parts aredenoted by the same reference characters and the duplicate descriptionthereof will be omitted.

Embodiment 1

FIG. 1 to FIG. 11 are views concerning the manufacturing technique of asemiconductor device having a thin film transistor as an embodiment(Embodiment 1) of this invention. Particularly, FIG. 1 to FIG. 9 areviews concerning the manufacture of a polycrystalline semiconductor thinfilm substrate and FIG. 10 and FIG. 11 are views illustrating a methodof manufacturing a thin film transistor by using the polycrystallinesemiconductor thin film substrate.

FIGS. 2( a)-(d) illustrate a method (process) of forming apolycrystalline semiconductor thin film of this Embodiment 1.

At first as shown in FIG. 2( a), an insulative substrate 602 (forexample, glass, fused quartz, sapphire, plastic and polyimide) is placedon a heating plate 606 (for example, carbon resistance heating heater).Subsequently, an amorphous semiconductor film 601 (for example, Si, Geand SiGe) is deposited on the insulative substrate 602 by using achemical vapor phase deposition method or a sputter deposition method onthe insulative substrate 602. The thickness of the amorphoussemiconductor film 601 is preferably 60 nm or less.

Then, as shown in FIG. 2( b), the heating plate 606 is heated to apredetermined temperature of 100° C. or higher. In this case, a careshould be taken so as not to cause unevenness in the temperature for theinsulative substrate 602 and the amorphous semiconductor film 601.

Then, the surface of the amorphous semiconductor film 601 is scanned bya first laser irradiation (first excimer laser irradiation) 604 (forexample, KrF or Xecl) in the direction of an arrow 603. The firstexcimer laser irradiation 604 is applied repeatedly for multiple timesat a suitable shape selection laser energy density Ec (360 mJ/cm²),which is to be set as described later. Further, after the first excimerlaser irradiation 604, second laser irradiation (second excimer laserirradiation) 605 is applied at a laser energy density lower than thesuitable shape selection laser energy density Ec (for example, 320mJ/cm²). For example, each of the first excimer laser irradiation 604and the second excimer laser irradiation 605 is applied by the number of30 to 60 cycles.

Now, the suitable shape selection laser energy density Ec set by thepresent inventors and the polycrystalline semiconductor thin film formedby the laser beam irradiation are to be explained.

FIG. 6 is a group of graphs showing the result of analysis such as forthe properties of the crystal grains and the difference of themanufacturing conditions in the manufacture of a polycrystallinesemiconductor thin film, in which FIG. 6( a) is a graph showing acorrelation between the shape (N) and the number density of crystalgrains, FIG. 6( b) is a graph showing a correlation between (N) and thefull width at half maximum and FIG. 6( c) is a graph showing acorrelation between the laser energy density and the formed shape (N).

FIG. 7 is a graph showing a correlation between hexagonal crystal grainsand the number of laser beam irradiation cycles, and a graph showing arelation between the number of laser beam irradiation cycles and theroughness on the surface of the polycrystalline semiconductor thin filmin the manufacture of the polycrystalline semiconductor thin film.Examples of crystal grains according to the existent method ofmanufacturing a polycrystalline semiconductor thin film by crystallizingan amorphous semiconductor film by laser heating includes trigonal,tetragonal, pentagonal, hexagonal, heptagonal and octagonal shapes asdescribed above. Their distribution is as shown in FIG. 6( a). In thegraph N is taken on the abscissa and the number density is taken on theordinate.

N is a number of closest crystal grains for an optional crystal grain.When the statistical distribution is examined for N, it forms a normaldistribution as shown in FIG. 6( a). The full width at half maximum ofthe normal distribution corresponds to the uniformess of thepolycrystalline film in which the polycrystalline film is made moreuniform as the full width at half maximum is narrower.

FIG. 6( b) shows a relation between the full width at half maximum andN. The full width at half maximum takes a minimum value at N=6. N=6 isequivalent with the surface shape of the crystal grains (the optionalcrystal grain) being hexagonal.

FIG. 6( c) illustrates a relation between the laser energy density and Nwhen the amorphous semiconductor film is laser heated at 100° C. orhigher. N is 6 when the energy density is Ec. That is, it has been foundthat the crystal grains tend to form hexagonal crystal grains when thelaser energy density is Ec. Then, Ec is defined as the suitable shapeselection laser energy density Ec.

FIG. 7( a) illustrates that the optimum number of laser irradiationcycles M is Mc when laser beam irradiation is applied at the laserenergy density of Ec or lower at a temperature of the amorphoussemiconductor film of 100° C. or higher.

FIG. 7( b) illustrates a relation between the number of laser beamirradiation cycles M and the roughness on the surface of thepolycrystalline semiconductor thin film in a case where the laserirradiation (second laser irradiation) is applied at a laser energydensity of Ec or lower at a temperature of the amorphous semiconductorfilm of 100° C. or higher. The roughness is defined as a top-to-bottommaximum length. Specifically, the top-to-bottom length at the corner ofthe grain boundary corresponds to this. Along with increase in M, theroughness decreases to 5 nm or less. Further, chemical bonds in thegrain boundaries are bonded again.

FIGS. 8( a)-(c) is a view illustrating the difference in the shape ofcrystal grains in the polycrystalline semiconductor film due to thedifference of the laser beam density. The view is obtained based onmicroscopic photography.

FIG. 8 (a) illustrates the state of crystal grains in thepolycrystalline semiconductor film at a laser energy density lower thanthe suitable shape selection laser energy density Ec. As can be seenfrom the figure, while crystal grains contain many hexagonal shapes,trigonal, tetragonal or pentagonal shapes are also present. Thehexagonal crystal grains are about 30 to 40% or less.

FIG. 8 (b) is an example of forming a polycrystalline semiconductor thinfilm while applying laser beam irradiation repeatedly at a suitableshape selection laser energy density Ec. This is an example of applyingthe first laser irradiation at the suitable shape selection laser energydensity Ec by multiple cycles and then applying the second laserirradiation at a laser energy density lower than the suitable shapeselection laser energy density Ec repeatedly. For example, the laserenergy density in the first laser irradiation is 360 mJ/cm² (suitableshape selection laser energy density Ec) and the laser energy density ofthe second laser irradiation is 320 mJ/cm². In this example, thehexagonal crystal grains can be formed by about 50 to 100% by increasingthe number of cycles of repeating irradiation. Further, the hexagonalcrystal grains are made uniform with a diameter of about 0.2 to 0.3 μm.

It was confirmed that 50 to 100% of the surface shape in the crystalgrains constituting the polycrystalline semiconductor thin film ishexagonal, for example, by scanning electron microscopic observationwhile taking a square evaluation region of 10 μm size on one sideincluding the center of the polycrystalline semiconductor thin film. Theresult of the observation for the evaluation region reflects the stateof the crystal grains for the entire surface of the polycrystallinesemiconductor thin film.

In the first laser irradiation, when laser beam irradiation is appliedrepeatedly by once or a predetermined number of cycles, hexagonalcrystal grains are formed successively as crystal grains and,subsequently, they continue to rotate or move as shown in FIG. 9( a) andthen respective sides of adjacent hexagonal crystal grains meet eachother as shown in FIG. 9( b). Further, according to the laser beamirradiation, less impurities less-segregate to the boundaries of thecrystal grain and the carrier concentration of each of the crystalgrains becomes constant.

As shown in FIG. 9( a), even when smaller crystal grain 1001 is formedbetween the hexagonal crystal grains 251, it is joined with largehexagonal crystal grains 251 at the periphery and eliminated in the stepof the first laser irradiation and the step of the second irradiationapplied repeatedly.

Further, in the second laser irradiation step, laser beam irradiationmay be applied successively at the suitable shape selection laser energydensity Ec.

FIG. 8( c) is an example of applying the laser beam irradiation at alaser energy density higher than the suitable shape selection laserenergy density Ec in which remelting of crystals occurs and grainboundaries are re-bonded into large crystal grains in an island shape.

After the first laser beam irradiation, undulations 610 are formed onthe surface of the polycrystalline semiconductor thin film 640 to formgrain boundaries 611. The grain boundaries 611 include dangling bonds.Further, as shown in FIGS. 2( b)-(d), the grain boundaries are graduallynarrowed as 611, 621 and 631 and the surface roughness is alsoplanarized successively as 610, 620 and 630 along with increase in thenumber of cycles of the laser beam irradiation.

By the method as described above, a polycrystalline semiconductor thinfilm substrate 260 as shown in FIG. 3 can be manufactured. FIG. 4 is across sectional view showing a portion of a polycrystallinesemiconductor thin film substrate 260.

As can be seen also from the figure, a polycrystalline semiconductorthin film 640 with a planar surface is formed. Further, most of crystalgrains in the polycrystalline semiconductor thin film 640 are alsohexagonal crystal grains 251 as shown in FIG. 5. The hexagonal crystalgrains 251 can be 50 to 100% depending on the manufacturing method.

The excimer laser irradiation is to be explained with reference toFIG. 1. An excimer laser apparatus has an UV-lamp heating device 106 ata lower portion and has an excimer laser (excimer laser generator) 101at a corresponding upper portion. The UV-lamp heating device 106 and theexcimer laser 101 are controlled by a control device 107.

An insulative substrate (glass substrate) 602 having an amorphoussemiconductor film (amorphous silicon film) 601 at the upper surface isdisposed on the UV lamp heating device 106 and preliminarily heated bythe UV lamp heating device 106. Further, a laser beam (excimer laserbeam) 660 is irradiated from the excimer laser 101. Since a notillustrated stage for supporting the insulative substrate 602 is movedrelatively to the excimer laser 101, the excimer laser beam 660 can beirradiated to the entire region of the amorphous semiconductor film 601on the upper surface of the insulative substrate 602 to be formed into apolycrystalline semiconductor thin film.

In this Embodiment 1, the laser beam irradiation is applied for twosteps of the first excimer laser irradiation 604 and the second excimerlaser irradiation 605. In each of the steps, the laser beam irradiationis applied repeatedly for about 30 to 60 cycles. Further, the laser beamirradiation is applied in the first excimer laser irradiation 604 at thesuitable shape selection laser energy density Ec and the laser beamirradiation is applied in the second excimer laser irradiation 605 at alaser energy density lower than the suitable shape selection laserenergy density Ec. In the second excimer laser irradiation 605, it maybe applied at a constant laser energy density, or the laser beamirradiation may be applied while lowering the laser energy densitygradually in the course of irradiation.

According to the method of manufacturing the polycrystallinesemiconductor thin film substrate of this Embodiment 1, 50 to 100% ofthe crystal grains 250 of the polycrystalline semiconductor thin film640 comprise hexagonal crystal grains 251 and the grain size is uniformas 0.2 to 0.3 μm, so that it is possible to provide a polycrystallinesemiconductor thin film substrate with an improved carrier mobility μand with less variation in the carrier mobility μ in each of theregions. The carrier mobility μ can be enhanced, for example, to about200-300 cm²/V·s.

Further, since electron orbits in the grain boundaries on the surfaceand inside the polycrystalline semiconductor thin film are bonded, itcan provide an effect of making the carrier mobility constant andimproving the reliability of individual transistors.

Further, when the polycrystalline semiconductor thin film 640 is formed,since the laser beam is irradiated repeatedly, the roughness on thesurface of the polycrystalline semiconductor thin film 640 is reducedand a planar polycrystalline semiconductor thin film substrate can beprovided. For example, the roughness can be restricted to 5 nm or less.

Further, when the polycrystalline semiconductor thin film 640 is formed,since the laser beam irradiation is applied repeatedly for multiplecycles at the suitable shape selection laser energy density Ec mostpreferred for forming the hexagonal shape, hexagonal seed crystals aresuccessively formed in the amorphous semiconductor film and adjacenthexagonal crystal grains move to each other and are successively inclose contact with adjacent hexagonal crystal grains 251. Subsequently,since the laser beam irradiation is applied repeatedly for multiplecycles at a laser energy density lower than the suitable shape selectionlaser energy density Ec, less impurities segregate to the grain boundaryand the carrier concentration is made constant for each of the crystalgrains.

The atmosphere upon UV-lamp heating and laser heating in this embodimentmay be vacuum, inert gas (for example, argon, krypton or helium) ornitrogen gas.

Then, a method of manufacturing a thin film transistor is to beexplained. For example, as shown in FIG. 11( a), an insulative substrate(glass substrate) 602 having a polycrystalline semiconductor thin film(polycrystalline silicon film) 640 at the surface is provided. Theexample of FIG. 11 has a structure in which a silicon oxide film 651 isplaced as a buffer layer between the insulative substrate 602 and thepolycrystalline semiconductor thin film 640, different from thepolycrystalline semiconductor thin film substrate 260 shown in FIG. 3.While the buffer layer may be omitted, this Embodiment 1 is to beexplained for the method of manufacturing a thin film transistor in apolycrystalline semiconductor thin film substrate 260 having the bufferlayer.

As shown in FIG. 11( a), a photoresist film 670 is disposed selectivelyfor forming a channel region 672 of the transistor and, subsequently,phosphorus (P) is implanted into the polycrystalline semiconductor thinfilm 640 and annealing is applied to form an n-type impurity region 671.The impurity region 671 forms a source region or a drain region. Ifnecessary, predetermined impurities are doped to the polycrystallinesemiconductor thin film 640 in the step of forming the polycrystallinesemiconductor thin film.

Then, as shown in FIG. 11( b), selective etching is applied to extendimpurity regions 671 each of a predetermined length on both sides of thechannel region 672.

Then, as shown in FIG. 11( b), a silicon oxide film is formed over theentire region on the upper surface of the insulative substrate 602 toform a gate insulation film 673.

Than, as shown in FIG. 11( b), a gate 674 is formed above the channelregion 672.

Alternatively, the impurity regions 671 to form the source region andthe drain region may be formed by disposing a gate insulation film 673and forming a gate electrode 674 and then implanting phosphorus into thepolycrystalline semiconductor thin film 640 using the gate electrode 674as a mask without conducting the impurity diffusion as described above.

Then, as shown in FIG. 10, after forming an interlayer insulation film675 over the entire region of the upper surface of the insulativesubstrate 602, contact holes are opened to form electrodes (sourceelectrode, drain electrode) 676 to be connected with the impurity region671 or not illustrated gate wiring electrode. Further, although notillustrated, the transistor is covered with a passivation film and aportion of the passivation film is removed to expose an externalelectrode.

While only one transistor is illustrated in the figure, it is actuallyformed in plurality.

In the transistor according to this Embodiment 1, 50% to 100% of thecrystal grains of the polycrystalline semiconductor thin film 640comprise hexagonal crystal grains 251 and the grain diameter is uniformas 0.2 to 0.3 μm. Accordingly, when a transistor (TFT) is formed, thegrain boundaries in the channel region of silicon below the gateelectrode are reduced and the carrier mobility μ is improved andvariation in the carrier mobility is decreased for each of thetransistors. The carrier mobility μ can be enhanced, for example, toabout 200-300 cm²N·s.

Further, the variation in the density of the grain boundaries present inthe channel region of silicon below the gate electrode is reduced ineach of the transistors, and the threshold voltage V_(th) is madeuniform for each of the transistors. Variation in the threshold voltageV_(th) can be suppressed to 0.1 V or lower.

Further, since the laser beam is irradiated repeatedly when thepolycrystalline semiconductor thin film 640 is formed, the roughness onthe surface of the polycrystalline semiconductor thin film is reduced,and variation in the performance is decreased for each of the individualtransistors and less degradation occurs to attain longer life of thetransistor.

Further, when the polycrystalline semiconductor thin film 640 is formed,since the laser beam irradiation is applied repeatedly by multiplecycles at a suitable shape selection laser energy density Ec for formingthe hexagonal shape, hexagonal seed crystals are formed successively andadjacent hexagonal crystal grains move to each other and are in closecontact with adjacent hexagonal crystal grains successively.Subsequently, laser beam irradiation is applied repeatedly for multiplecycles at a laser energy density lower than the suitable shape selectionlaser energy density Ec. Accordingly, less impurities segregate in thegrain boundaries and the carrier concentrations are made constant foreach of the crystal grains. As a result, characteristics of thetransistor become stable.

Further, since crystal grains of uniform shape and size are formed inthe transistor channel region and chemical bonds of the grain boundariesare re-bonded to provide a surface with less roughness, the interfacedensity of state between the semiconductor and the gate insulation filmcan be lowered to lower the threshold voltage V_(th). Further, by thesame reasons as described above, variation due to short-channel can besuppressed.

In this embodiment, since the crystal grains in the polycrystallinesemiconductor thin film are formed as the hexagonal crystal grains ofuniform size, the carrier mobility is high and it varies less and thethreshold voltage V_(th) varies less. Accordingly, when pluraltransistors are manufactured, characteristics scatter less for each ofthe transistors and the manufacturing yield of semiconductor devices canbe improved. As a result, the manufacturing cost of the semiconductordevice can be reduced.

Embodiment 2

FIG. 12 is a schematic view illustrating the state of forming apolycrystalline semiconductor thin film as another embodiment(Embodiment 2) of this invention.

In the excimer laser device of Embodiment 2, as shown in FIG. 12, aninsulative substrate (glass substrate) 602 having an amorphoussemiconductor film (amorphous silicon film) 601 at the upper surface islocated between a UV-lamp heating device 106 in the lower portion and anexcimer laser (excimer laser generator) 101 in the upper portion, andthe UV-lamp heating device 106 and the excimer laser 101 are controlledby the control device 107. Preliminary heating is applied by the UV-lampheating device 106 and the amorphous semiconductor film 601 is formedinto a polycrystalline semiconductor thin film by a laser beam 160emitted from the excimer laser 101.

In Embodiment 2, the UV-lamp heating device 106 and the excimer laser101 are controlled by using the control device 107 to synchronize thelight emitting interval of the UV-rays and the excimer laserirradiation. In this case, thermally-induced strain formed between theinsulative substrate 602 and the amorphous semiconductor film 601 can becontrolled.

Embodiment 3

FIG. 13 is a schematic view illustrating the state of forming apolycrystalline semiconductor thin film as another embodiment(Embodiment 3) of this invention. Explanation is to be made,particularly, for the constitution of the excimer laser beam irradiationin FIG. 3.

A laser beam 110 emitted from an excimer laser 101 is irradiated to anamorphous semiconductor film 601 at the upper surface of an insulativesubstrate 602 placed on a sample stage 122. In this Embodiment 3, thelaser beam 110 emitted from the excimer laser 101 splits in two opticalchannels by an optical component such that one of them reaches a laserbeam irradiation position with a delay.

That is, the laser beam 110 emitted from the excimer laser 101 isdivided by a half mirror 102 into two optical channels in which one ofthem passes through a mirror 103 and a mirror 105 and reaches a laserbeam irradiation position, while the other is reflected at the halfmirror 102 and then directly reaches the laser beam irradiationposition.

With this constitution, a laser beam 112 passing through an opticalchannel of a shorter optical channel length can preliminarily heat theamorphous semiconductor film 601 and melts the amorphous semiconductorfilm 601 together with a laser beam 111 that reaches passing through anoptical channel of a longer optical channel length with a delay.

Then, laser beam irradiation is applied repeatedly for multiple cyclesat the suitable shape selection laser energy density Ec as the firstlaser irradiation step and, successively, the laser beam irradiation isapplied repeatedly for multiple cycles at a laser energy density lowerthan the suitable shape selection laser energy density Ec as the secondlaser irradiation step, thereby enabling to manufacture apolycrystalline semiconductor thin film substrate at a good quality inthe same manner as in the previous embodiments.

Embodiment 4

In Embodiment 4, an electronic apparatus incorporated with a transistor(thin film transistor) manufactured by the previous embodiment will bedescribed.

FIG. 14 is a schematic perspective view illustrating a portion of aliquid crystal display according to another embodiment (Embodiment 4) ofthe present invention.

Explanation is to be made in this Embodiment 4 to an example ofincorporating a semiconductor device 40 formed with plural transistors(thin film transistors) 18 to a polycrystalline semiconductor thin filmsubstrate 260 (amorphous semiconductor film 601 formed on the uppersurface of the insulative substrate 602) into an image display apparatus(electronic apparatus).

FIG. 14 is a perspective view in an exploded state illustrating aportion of the image display apparatus. As shown in FIG. 14, it isconstituted such that liquid crystals are disposed on a semiconductordevice 40 in which a group of transistors are formed to the uppersurface of a polycrystalline semiconductor thin film substrate 260 and adisplay panel 22 constituting pixels 23 is stacked andglass-encapsulated. Transistors 18 as the pixel driver correspond toeach of the pixels 23, and the source electrode of the transistor 18 andthe pixel electrode of the pixel 23 are connected to each other bystacking.

Peripheral driver circuits 19 such as an address decoder, adigital/analog conversion circuit and a controller are disposed to theperiphery out of the region in which the pixels 23 are arranged.Reference numerals 10 and 21 show transistor forming regions.

In the electronic apparatus described above, since the size of thecrystal grains is uniform in the channel region of each of thetransistors 18 corresponding to the pixel 23, the carrier mobility isconstant and the threshold voltage V_(th) is also constant as describedin Embodiment 1, image display at high performance is possible andreliability can be improved in an image display of a large area.

Embodiment 5

FIG. 15 is a schematic perspective view illustrating a portion of a dataprocessor according to another embodiment (Embodiment 5) of thisinvention and FIG. 16 is an enlarged schematic view of a portionsurrounded with a dotted chain circle in FIG. 16.

Also in this Embodiment 5, crystal grains to be used are formed on thesurface of an insulative substrate 602, that is, a glass substrate toform a polycrystalline semiconductor thin film substrate 260 by the samemethod as in Embodiment 1.

A data processor 30 comprises each of circuitries formed on the surfaceof a polycrystalline semiconductor thin film substrate 260. That is, asshown in FIG. 15, transistors 18 and passive elements not shown areformed on the surface of the polycrystalline semiconductor thin filmsubstrate 260. Further, each of the circuitries is connected withwirings not shown and has such a structure in which external terminalsare disposed on the surface of the polycrystalline semiconductor thinfilm substrate 260 or connectors are attached to the edges thereof.

Further, each of the circuit portions and wirings on the surface of thepolycrystalline semiconductor thin film substrate are covered andprotected with a passivation film. Reference numeral 10 denotes atransistor film region.

The data processor 30 comprises, for example, a central processing unit24, a memory circuitry 26, an input/output controller 28 and aperipheral circuitry 27 connected respectively by way of a data buscircuitry 29 to the central processing unit 24, and a cash circuitry 25connected with the central processing unit 24.

In the data processor 30, each of the transistors is formed into apolycrystalline semiconductor thin film. Since each of the transistorsis formed as a polycrystalline semiconductor thin film with the uniformsize of the crystal grains and with uniform grain boundaries in thechannel region, the carrier mobility μ is high with less variation andalso the variation in the threshold voltage V_(th) is small.

Accordingly, the electric field effect mobility is higher than thatformed in the existent polycrystal semiconductor thin film and themanufacturing cost of the data processor 30 can also be reduced.

The inventions made by the present inventors have been explainedconcretely with reference to the preferred embodiments but it will beapparent that the inventions are not limited to the embodimentsdescribed above and can be modified variously within a scope notdeparting the gist thereof.

Further, in the foregoing explanations, although the inventions made bythe present inventors have been described in the case where theinvention is applied to the image display or the data processor in thefield of use as the background of the invention, it is not limitedthereto and is applicable also to other electronic devices.

These inventions are applicable at least to electronic devices that canbe manufactured by using the polycrystalline semiconductor thin film.

The effects obtainable by typical inventions among those disclosed inthe present application will be simply explained as below.

(1) In the polycrystalline semiconductor thin film substrate, thecrystal grains of the polycrystalline semiconductor thin film can beformed into the hexagonal crystal grains of a uniform size and the ratioof the hexagonal crystal grains can be 50 to 100%.

(2) In the polycrystalline semiconductor thin film substrate, it ispossible to provide the polycrystalline semiconductor thin filmsubstrate in which the size and the carrier concentration are uniformand the surface is planar.

(3) A semiconductor device having the thin film transistor of favorablecharacteristics and with less variation in characteristics can beprovided.

(4) A semiconductor device having the thin film transistor of highcarrier mobility and with less variation in characteristics can beprovided.

(5) The yield of the semiconductor devices can be enhanced and reductionin the manufacturing cost for the semiconductor device can be attained.

(6) Electronic apparatus such as a liquid crystal display and a dataprocessing unit at favorable high speed performance can be provided.

1. A semiconductor device comprising plural transistors formed in apolycrystalline semiconductor thin film, wherein the polycrystallinesemiconductor thin film is formed by a plurality of laser irradiationsteps, wherein the laser irradiation steps are carried out so that,after the last laser irradiation step, the number of crystal grains withthe number of closest crystal grains of 6 is greatest among pluralcrystal grains that form the polycrystalline semiconductor thin film. 2.A semiconductor device as defined in claim 1, wherein the roughness ofthe grain boundaries on the surface of the polycrystalline semiconductorthin film is 5 nm or less.
 3. A semiconductor device comprising pluraltransistors formed in a polycrystalline semiconductor thin film, whereinthe polycrystalline semiconductor thin film is formed by a plurality oflaser irradiation steps, wherein the laser irradiation steps are carriedout so that, after the last laser irradiation step, the number ofcrystal grains with the number of closest crystal grains of 6 isgreatest among plural and non-uniformly oriented crystal grains thatform the polycrystalline semiconductor thin film.
 4. A semiconductordevice a defined in claim 3, the roughness of the grain boundaries onthe surface of the polycrystalline semiconductor thin film is 5 nm orless.
 5. A semiconductor device as claimed in claim 3, wherein thecrystalline orientations are independent of the shape of crystallinesurfaces of the plural crystal grains that form the polycrystallinesemiconductor thin film.
 6. A semiconductor device as claimed in claim1, wherein the plural crystal grains that form the polycrystallinesemiconductor thin film have crystalline orientations independent of thecrystalline surface shapes.